The defining feature of wireless communication networks is user mobility. That is, users may wander freely, while maintaining connectivity to the network, and without experiencing interruptions in voice or data transfer. As known in the art, at any one time a User Equipment (UE) accesses a network, and receives services, from an access node (sometimes referred to as a cell, which also connotes the geographic area served by an access node). Handover is a vital aspect of user mobility. Handover is the process of transferring an ongoing connection of a UE from one access node (the serving node or source node) to another access node (the target node) in order to accomplish a transparent and seamless continuity of service over a large geographic area. The handover should happen without any loss of data and with as small an interrupt as possible.
FIG. 1 depicts a wireless communication network 10 comprising core network (known in the 3GPP Long Term Evolution, or LTE, protocol as an Evolved Packet Core, or EPC) 12 and a Radio Access Network (RAN) 14. The RAN 14 comprises a plurality of base stations (known as eNodeB or eNB in LTE) 16, 18, 20. The eNBs 16, 18, 20 are referred to more generally herein as access nodes 16, 18, 20 of the network 10. Access node 16 provides service to a UE 22, and is referred to herein as the serving node 16. The UE 22 is also within range of access nodes 18, 20, which are candidates for a handover of the UE 22, and are hence referred to herein as candidate nodes 18, 20. All of the access nodes 16, 18, 20 connect to a Mobility Management Entity (MME) 24 and Serving Gateway (SGW) 26 of the core network 12. As known in the art, the core network 12 includes numerous additional nodes, and provides communication and data connectivity to numerous other networks (not shown).
Prior to initiating a handover procedure, it is necessary to ascertain a suitable target node 18, 20, and to ensure that it is possible for the UE under consideration to sustain reliable communication with that target node 18, 20. Candidates for suitable target node 18, 20 are usually stored in so-called neighbor lists, which are stored at, or at least are accessible by, the serving node 16. To ensure that it is possible to sustain reliable communication with a target node 18, 20, the connection quality in the target cell must be estimated before the handover procedure is initiated. Additionally, before a handover is initiated, a precise estimate of the timing difference between the UE 22 and the target node 18, 20 must be obtained, so that a Timing Advance (TA) value may be calculated and transmitted to the UE 22, allowing the UE 22 to be subframe-synchronized to target node 18, 20.
The connection quality and timing in the target node 18, 20 are estimated by measurements related to the UE 22. Either or both of downlink and uplink measurements can be considered. In legacy systems, handover has been most commonly based on downlink measurements. This is a natural solution, as all access nodes 16, 18, 20 continuously transmit pilot signals, which UEs 22 in neighboring cells can use to estimate, e.g., the target cell connection quality. This is true in the Global System for Mobile Communications (GSM) (using the Broadcast Control Channel, or BCCH), Wideband Code Division Multiple Access (WCDMA) (using the Common Pilot Channel, or CPICH) and Long Term Evolution (LTE) (using Cell-specific Reference Signals, or CRS) as well as in IEEE 802.11 Wireless Local Area Networks (WLAN) such as Wi-Fi (using the beacon frame). Using these downlink signals, a UE 22 may estimate the quality of neighbor cells with relatively good accuracy.
Modern and future cellular systems will use advanced antenna systems to a large extent. With such antennas, signals will be transmitted in narrow beams, to increase signals strength in some directions, and/or to reduce interference in other directions. Continuously transmitting pilot signals in all of these beams is undesirable, since it will generate significant interference; increase the base station energy consumption; and will not serve the function of facilitating the building of neighbor lists, since the narrow, targeted beams will not be received generally by all UEs 22 in the cell and in neighboring cells.
Accordingly, in systems with advanced antennas and sophisticated beamforming, it becomes much more natural to rely on uplink measurements to assess channel quality and timing for potential handover. For fifth generation (5G) networks, defined, e.g., according to the 5G New Radio standard, an uplink measurement-based mobility approach may be devised where an uplink measurement signal is transmitted by the UE 22, similar to the Physical Random Access Channel (PRACH) signal design.
FIG. 2 depicts a functional view of a PRACH reception model, as disclosed by Henrik Sahlin, et al., in the paper “Random Access Preamble Format for Systems with Many Antenna,” published in Globecom 2014, the disclosure of which is incorporated herein by reference in its entirety. The PRACH signal comprises a number of repetitions of a short sequence (e.g., a Zadoff-Chu sequence). Unlike the legacy LTE PRACH model, in which the PRACH is a single, long OFDM symbol (much longer than data symbols), in this model the PRACH signal comprises a series of normal-length symbols—that is, the same symbol length as is used for data transfer on PUSCH, etc. At an access node receiver, each symbol in the PRACH subframe is individually converted to the frequency domain, and matched filters are applied to detect (potentially one of several) PRACH sequences. Matched filter outputs are accumulated over the subframe, and an Inverse Fast Fourier Transform (IFFT) is applied. At the IFFT output, a time-domain peak appears—the delay of which corresponds to a timing error between the UE and the access node receiver. The signal design allows reception and timing estimation with only rough initial timing alignment; the timing accuracy must be better than one Orthogonal Frequency Division Multiplexing (OFDM) symbol length. If the misalignment exceeds one OFDM symbol, estimation returns an incorrect timing offset estimate. The key to this approach is that each PRACH symbol repeats the same sequence; hence the receiver accumulates a sufficient number of symbols to acquire enough energy to perform the correlation and timing detection.
In a handover situation, an uplink measurement signal designed to facilitate handover timing, which may be similar to the PRACH design, would be transmitted by the UE 22 using download timing with respect to its source node 16. Since the received uplink measurement signal at a candidate node 18, 20 is not necessarily aligned with symbol and subframe boundaries to within the cyclic prefix, the uplink measurement signal will “spill over” into symbol periods in which reception of user data symbols has been scheduled. These user data cannot be received without interference from the uplink measurement signal, and vice versa. Therefore, for candidate nodes 18, 20 to detect and measure the uplink measurement signal from the UE 22, they must reserve all uplink symbol periods during any subframes where the uplink measurement signal is expected (in the relevant frequency subband, if the full uplink carrier is not used).
In cases where the timing misalignment between the source node 16 and a candidate node 18, 20 exceeds one OFDM symbol (e.g., 14 μsec @ 75 kHz numerology), misalignment at the candidate node subframe reserved for uplink measurement signal reception leads to uplink measurement signal detection performance degradation. The timing misalignment may be due to propagation delay. For example, in rural deployments, a distance of >4 km will introduce such an error. Additionally or alternatively, the timing misalignment may be due to inter-node synchronization inaccuracy.
If the nominal single-subframe uplink measurement signal allocation is used at a misaligned candidate node 18, 20, only a part of the transmitted uplink measurement signal will fall within the allocated subframe. In this case, part of the transmitted uplink measurement signal energy is not utilized and measurement quality is reduced. Furthermore, the misaligned part of the uplink measurement signal may interfere with reception of user data from other UEs 22, as it is received in a subframe allocated to user traffic.
One straightforward solution would be to reserve all subframes containing uplink measurement signal symbols for uplink measurement signal reception and measurement. However, in this case resource utilization efficiency suffers. Reservation of two subframes means that only 50% of the reserved time window will actually be utilized for uplink measurement signal reception. The other 50% during the uplink measurement signal subframes in non-aligned candidate nodes will remain unused, thus reducing the capacity of the network. The capacity loss may be significant since the extended uplink measurement signal reception window may be applied at many candidate nodes, at many uplink measurement sessions, for many UEs 22.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.